The invention relates to a thin-film heating element comprising a temperature-stable, electrically insulating substrate having a thin electrically conductive metal oxide film which is doped with pairs of compensating foreign atoms and each of which pairs consist of at least one acceptor-forming element and one donor-forming element, the metal oxide film being provided with connecting electrodes.
An acceptor is a local impurity in a semiconductor, which either accepts an electron or supplies a mobile hole. The corresponding electronic energy level is situated in the forbidden band, the exact location together with the capture cross-section of the electrons determining the operation of the acceptor. When acceptors are used as dopants, the host lattice atom is replaced by an atom having one valence electron less than the host lattice atom. A donor is an impurity in a semiconductor, which can give up one of its electrons. The corresponding electronic energy level is situated in the forbidden band, the operation of the donor being determined by the exact location and the capture cross-section of the electrons and mobile holes. When donors are used as dopants, a host lattice atom is replaced by an atom having one valence electron more than the host lattice atom.
It is known from, for example, U.S. Pat. No. 3,108,019 that electrically conductive, thin metal oxide films on a temperature-stable, electrically insulating substrate are used as resistance elements in heating devices such as, for example, heated windows (for example car windows), warming trays or similar devices, these thin layers being used as heating elements in a temperature range up to 500.degree. C. For this purpose, glass substrates or ceramic substrates are coated in a pyrolytic deposition process from solutions containing, for example, the chlorides, bromides, iodides, sulphates, nitrates, oxalates or acitates of tin, indium, cadmium, tin and antimony, tin and indium or tin and cadmium with or without a dopant such as tin, iron, copper or chromium. The films formed by pyrolytic deposition then consist of the corresponding metal oxide(s).
In certain applications, thin-film heating elements which can attain surface temperatures exceeding 500.degree. C. are preferably used.
For the sake of completeness, it is pointed out that thin electrically conductive indium oxide films are known from U.S. Pat. No. 2,564,709, which are doped with foreign atoms in a quantity up to 10 at %, which atoms compensate each other in pairs and which each consist of at least one acceptor-forming element and one donor-forming element, the quantities of the acceptor-forming elements and the donor-forming elements, however, differing more than 10%. This known coating material has proved to be insufficiently stable at higher surface temperatures.